This invention relates to the manufacture of an automotive catalytic converter comprising cold-rolled aluminum-containing steel foil having an oxide whisker-covered surface and carrying a catalyst-impregnated alumina coating. More particularly, this invention relates to growing dense alumina whiskers on a surface of cold-rolled Fe-Cr-Al or Fe-Cr-Al-Y alloy foil for tightly bonding an alumina coating.
U.S. patent application Ser. No. 098,075, filed Nov. 28, 1979, of common assignee-ownership, describes the manufacture of a monolith-type catalytic converter for treating automotive exhaust gases. The converter comprises a steel foil suitably wound to form a structure having gas-conveying passages. The foil is composed of an iron (Fe)-base alloy containing aluminum (Al), chromium (Cr) and preferably yttrium (Y); and commonly designated Fe-Cr-Al or Fe-Cr-Al-Y. The high temperature corrosion resistance of the stainless steel alloy, particularly the yttrium-containing alloy, make it preferred for converter use. The foil is fabricated by a metal peeling process and heated in air to grow high aspect alumina whiskers that substantially cover the surfaces. Thereafter, the whiskered surfaces are coated with a gamma alumina material and the coating is impregnated with a noble metal catalyst.
The alumina whiskers substantially improve adhesion of the alumina coating to the metal foil. This reduces spalling of the coating during converter use, which involves mechanical vibration and temperature cycling, the latter being particularly harmful because of the difference in thermal expansion between the alloy and the coating. The high density of alumina whiskers over substantially the entire surface is attributed to the high density of metal defects produced by the peeling process. In contrast, cold-rolling produces a foil having a relatively defect-free and smooth surface, which grows a flat or slightly nodular oxide. Even when subjected to the preferred whisker growing treatment for peeled foil, the cold-rolled foil forms only occasional, if any, whisker formations. However, Fe-Cr-Al or Fe-Cr-Al-Y alloy is difficult to obtain in billet form suitable for peeling, and it is desirable to manufacture the converter out of cold-rolled foil, which is readily commercially available.
Therefore, it is an object of this invention to provide a method for growing oxide whiskers on cold-rolled metal foil composed of a ferritic stainless steel alloy. The preferred alloy is predominantly iron and contains aluminum, chromium and optionally yttrium. The method comprises first treating the foil to produce a whisker-precursor surface and thereafter growing densely spaced whiskers that substantially cover the surface.
It is also an object of this invention to provide a nonpeeled, wrought foil composed of Fe-Cr-Al alloy and having an integral, protective oxide layer that features densely spaced alumina whiskers and is capable of tightly bonding an applied coating. In a particularly useful aspect, a catalyst-bearing alumina coating is applied to the whisker-covered foil and the foil is formed into an automotive catalytic converter structure.
It is a more particular object of this invention to provide a method for preparing a Fe-Cr-Al alloy foil surface to bond a ceramic coating with improved adhesion, which method comprises initially heating the metal surface in an oxygen-deficient atmosphere and thereafter heating in an oxygen-rich atmosphere to grow high aspect alumina whiskers. The whiskers substantially cover the surface and tightly bond the subsequently applied coating.
It is a further object of this invention to provide an improved monolith-type catalytic converter for treating automotive exhaust gases and formed of a cold-rolled Fe-Cr-Al or Fe-Cr-Al-Y alloy foil. The foil comprises an oxidized surface substantially covered with high aspect, densely spaced alumina whiskers. An alumina material is applied to the surface and impregnated with an effective catalyst. The whiskers anchor the coating to reduce spalling during exhaust treatment.